1. Technical Field
This invention generally relates to drilling strings used to bore wells in the earth and, more particularly, to a non-metallic pipe for drilling a deviated well bore.
2. Background Art
As is generally known, underground petroleum reserves, water, and soluble minerals often are accessed through vertical bores which extend downwardly through the earth. To create a vertical bore, a rotating metal tube having a leading bit or cutting element is penetrated through the crust of the earth's surface and is advanced downwardly until the tube reaches an oil bearing formation. The drilling string then is withdrawn from the bore and oil or other desired substances are pumped upwardly from the formation and through the bore with known extraction techniques.
In an environment where a substance reserve cannot be accessed with a conventional straight vertical well bore, directional drilling techniques are used to create a deviated or extended-reach bore. Generally, a flexible rotary drilling string is inserted through the vertical portion of an existing straight well bore. When the leading end of the drilling string reaches a predetermined depth, the drill bit is deviated from the straight path and burrowed through the sidewall of the existing well bore. The drilling string continuously is advanced to create a relatively short-radius curved bore having a substantially horizontal passage which can extend hundreds of feet to reach an otherwise inaccessible oil bearing formation, water supply, or mineral reserve.
It may be understood from the foregoing that in order to create a suitable deviated well bore a drilling string must be sufficiently flexible to permit continuous angular displacement of the string while, at the same time, providing adequate structural integrity to withstand the compression force and bending load which are induced when the string is advanced through the earth. Moreover, while a drilling string must be ductile enough to accommodate overall angular deformation in excess of ninety degrees, the string must also be capable of enduring the abuse inherent in the downhole environment.
Attempts have been made at providing an acceptable drilling string by connecting a number of short metal tubes, such as the articulated drill pipe shown in U.S. Pat. No. 4,685,985 to Hatten, issued Aug. 11, 1987 (see FIG. 1). The Hatten drill pipe, generally designated 10, has a plurality of interlocked metal "jigsaw puzzle" pipe sections 12 which are lowered through the straight portion of a substantially vertical well bore 14. A cutting assembly 16 at the leading end of the drill pipe then is deviated from the straight path to create a curved bore 18. These types of drilling strings have a number of problems.
Specifically, the articulated connection between successive pipes is achieved with a dove-tail joint or a hinge Joint. Articulated joints permit excessive and erratic movement of the articulated pipe sections which, in turn, requires frequent directional measurements and appropriate corrections to the orientation of the bit to drill along a desired path. In addition, the joints continuously open and close as the pipe rotates. Cuttings can enter the openings and induce significant joint wear and degradation.
Metal drill pipes are very heavy and awkward to handle, making the installation and operation of the drill pipe both cumbersome and dangerous. The extreme weight of metal drill pipes produces large friction forces when the pipe is rotated about an axis off vertical, such as when the pipe is used to drill a horizontal bore. The friction forces induce excessive wear of the articulated pipe and thus limit the horizontal distance which can be drilled. Moreover, the geometry of the "jigsaw puzzle" piece lobes in a dove-tail joint may produce high stress concentrations which result in fatigue failure after an unacceptably few number of cycles.
The present invention is directed toward overcoming one or more of the problems set forth above.